quote:
2) The fact that carbon was elected, arbitrarily, at an assumed composition of exactly 12 atomic mass units ignored the carbon isotopes. Compounding this, all elements have atomic weights, in amu, which also have been averaged with their isotopes. Hence, even though any sample must contain, say (x) number of whole particules, we are content with factional averages, implying that the Na is not reaaly a head count.
It is not an assumed composition. It's a direct definition of u. You are correct in that we could chose more or less any other value of u, but it's been defined this way and as such is very much definite.
The isotope spread is statistical, and therefore can't be exact for all numbers. Na is a head count, but you can't always be sure, in a given practical situation, the exact spread of isotopes. But of course we don't get factions of atoms.
[This message has been edited by Melchior, 04-25-2004]
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